The present invention relates generally to sprue bushings for injection molding machines and, more particularly, to internally-heated sprue bushings having an integral construction and an internal heater unit having a highly thermally conductive material surrounding an internal heater. In this regard, one important aspect of the present invention is directed to an internally heated bushing assembly which is especially suited to handle pressurized melt, which assembly includes a unitary bushing casing-core element having an integral outer sidewall, endwall and inner core having a melt running passage therethrough.
Sprue bushings are commonly used in injection molding to provide a heated flowpath for conveying molten plastic or melt from an injection molding machine manifold or nozzle to an injection mold cavity opening. Typically, these sprue bushings may include a separate bushing casing portion housing a heating element and a separate, inner bushing core portion through which the melt flows. The bushing core is usually disposed in the center of the bushing casing and extends axially through the casing between the bushing inlet and an outlet end thereof with one end of the core projecting through a hole in the casing. Other sprue bushings such as that shown in U.S. Pat. No. 4,882,469 have an integral casing-core construction wherein the casing and core are formed from one piece with an annular cavity formed therebetween which accommodate a preformed tubular heater element. In either of such sprue bushing constructions, the inner core is surrounded by a tubular or spiral heating element set in a particulate, compacted refractory material. The refractory material insulates the heating elements and conveys or transmits heat from the heating element(s) to the center core portion of the bushing.
Although the refractory material transmits heat adequately to the bushing center core, it is desirable to increase the effectiveness of heat transfer from the heating element to the inner core. This is accomplished in the present invention by surrounding the heating means disposed in the bushing inner core with a highly thermally conductive material, such as copper or an alloy thereof. The surrounding layer of the highly thermally conductive material provides for efficient heating of the inner core melt runner passage for substantially the entire length thereof. Significantly, this surrounding thermal layer, or cladding, may extend between the point where the bushing inner core meets the head member at the melt inlet of the sprue bushing and the rear inner wall of the bushing casing near the bushing outlet. The transfer of heat through the surrounding thermal layer of thermally conductive material is effectively accomplished along substantially the entire length of the center core, thereby effectively improving the heat distribution along the melt runner passage.
The present invention provides the above-mentioned benefits and is directed to a novel sprue bushing assembly in which the bushing inner core or central melt runner passage has a heating element disposed therein with a surrounding layer of a highly thermally conductive material applied thereto, such as copper. As such, these sprue bushing assemblies are characterized by a unique bushing casing-core element which includes an unitary casing having an outer sidewall, endwall and elongated inner core integrally joined together, the inner core having a melt runner passage extending therethrough which is heated by a heating element surrounding the inner core and embedded in a surrounding layer of copper.
In a sprue bushing assembly incorporating the principles of the present invention, a cylindrical metal blank is provided with a cavity axially extending within the blank. An elongated inner core member, which is integrally formed with the bushing casing endwall is spaced apart from the outer wall of the bushing casing. The cavity contains at least one heating element in the form of a coil heater which encircles the inner core from between the bushing inlet and the bushing outlet. The open, surrounding space which occurs in the casing cavity between the heating element and the bushing is filled with a powdered or particulate thermally conductive material to form a heater-bushing blank assembly. When filled, the heater-bushing blank assembly may then be heated until the particulate thermally conductive material melts and solidifies in the casing cavity, thereby forming all of the components which fill the casing cavity, i.e., the heating element and thermally conductive material, into a unified mass which is substantially free of voids. A head member containing electrical conduction and temperature sensing wires to the heating element is then added to the top portion of the bushing-heater assembly and the cap then firmly welded to the bushing assembly. An opening is drilled through the bushing core to provide a melt flowpath. The outlet end of the bushing is then finished to form a final bushing outlet tip.
Accordingly, it is a general object of the present invention to provide a method of manufacturing an improved sprue bushing from a single metal blank having improved heat transfer means operatively associated with the inner core and wherein the inner core is integral with the bushing casing which eliminates melt leakage to the heating element and resulting heater element burnout.
Another object of the present invention is to provide an injection molding sprue bushing in which the bushing core has a heating element disposed therein which is surrounded by a layer of thermally conductive material for substantially the entire length of the bushing core.
A yet further object of the present invention is to provide an internally heated sprue bushing having an inner core encircled by a heating element encased in and in which a thermowell is disposed in contact with the inner core.
Still another object of the present invention is to provide a sprue bushing for injection molding having an outer bushing casing, an inner core axially extending through the bushing casing, the inner core having two heating elements surrounding it and disposed longitudinally within an annular cavity formed between the inner core and bushing casing, the heating element(s) being surrounded by a highly thermally conductive material.
These and other objects, features and advantages of the present invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings wherein like reference numerals refer to like parts.